1. Field of the Invention
The invention relates to a process and apparatus for preparing solvent-activated amorphous particle dispersions in a continuous process.
2. Information Disclosure
Photographic oil-in-water dispersions are commonly used for the incorporation of materials such as dye-forming couplers, oxidized developer scavengers, dyes and other compounds. Conventional techniques involve dissolving such compounds in organic solvents at high temperatures followed by emulsification of this oil solution into an aqueous gelatin/surfactant solution to form a dispersion. Solvents are used both to facilitate the dissolving of compounds and also to improve photographic activity of compounds by plasticizing the compound, thereby increasing the molecular mobility and lowering the particle bulk glass transition. In some cases, no-solvent (NS) dispersions are prepared by the same approach followed by removal of the solvent by evaporation or washing techniques. The NS dispersion is then considered an amorphous particle dispersion.
Techniques are known for the forming of amorphous-particle dispersions of compounds without the use of solvents. Such techniques are classified as either condensation or dispersion techniques. Modulated Phase Separation (MPS), which is described in U.S. Pat. Nos. 5,104,776 (Bagchi and Sargeant) and 4,970,139 (Bagchi), involves dissolving couplers with acid groups in aqueous alkaline solutions followed by precipitation by reprotonation in the presence of a suitable wetting agent. The resultant dispersion particles are characterized as amorphous and have a size range of 50 to 300 nm.
An alternative technique is disclosed in U.S. Pat. No. 5,110,717 (Czekai) which is incorporated herein by reference. This technique, called a Thermally Modified Solid (TMS) dispersion process, involves preparation of a microcrystalline dispersion of the crystalline compound in a non-solvent liquid (usually water) with suitable wetting agents. The amorphous dispersion is obtained by mechanical fragmentation followed by heating or pressurized heating of the dispersion to the melting temperature of the dispersed crystals, at which temperature the microcrystals undergo a phase change and are converted to amorphous particles of a size range of 10-500 nm. These amorphous particles have properties that render them more desirable than microcrystalline particles for certain photographic uses; therefore, recrystallization subsequent to the formation of the amorphous particles is to be avoided.
In the case where the amorphous particles are dye forming couplers, it is known that the addition of a separate non-aqueous solvent to the pre-formed aqueous coupler dispersion results in increased photographic activity. This phenomenon has been observed for conventional NS (no-solvent) dispersions and for MPS dispersions. The solvent is thought to partition by diffusion through the aqueous phase to the coupler particle which it then plasticizes. The increased photographic activity lasts only as long as the particles remain amorphous, and unfortunately the solvent not only plasticizes the amorphous particles, it also increases their tendency to recrystallize.
One advantage of the solvent-free, aqueous amorphous particle dispersion is the improved stability against recrystallization of the amorphous particles. In general, in the absence of non-aqueous solvent, the dispersions retain their sensitometric properties for relatively long periods of storage. However, once a solvent is introduced, storage stability declines sharply. The prolonged presence of solvent in the amorphous particle dispersion appears to increase susceptibility of the plasticized particle to recrystallization. There is a "window" of time during which the photographic activity of amorphous particle dispersions is improved by the presence of solvent. This is a particular problem in coating delivery processes where the dispersion is held at elevated temperatures for many hours.
There is thus a need for an in-line process that would produce a solvent-activated amorphous particle dispersion in a continuous mode.
It is therefore an object of the invention to provide a process that can be used to produce an optimally active amorphous particle dispersion.
It is a further object to provide a process that can be run continuously, rather than batchwise.
It is a further object to provide a process for amorphous particle dispersions that minimizes recrystallization of plasticized particles.
It is a further object to provide an in-line process for the addition of solvents to amorphous particle dispersions that eliminates the need for a separate solvent dispersion manufacturing operation.
It is a further object to provide an in-line process that enables the automated, continuous delivery of dispersion, gelatin and solvent solutions to coating processes, thereby improving productivity.